High temperature plasma confinement using a travelling electromagnetic field



April 10, 1962 K. G. HERNQvlsT HIGH TEMPERATURE PLASMA CONFINEMENT USING A TRAVELLING ELECTROMAGNETIC FIELD 5 Sheets-Sheet 1 Filed Aug. 19, 1958 INVENTOR. KARL E. HERNqvIs-r irme/vir April 1o, 1962 K. G. HERNQvxs-r 3029,361 HIGH TEMPERATURE PLASMA CONFINEMENT USING A TRAVELLING IELD Filed Aug. 19, 1958 3 Sheets-Sheet 2 ELEc'rRoMAGNE'rIc F T Rm O H. .www mm 5" m A 7 Kw Irfan/fr Aplll 10, 1962 K. G. HERNQvlsT 3,029,361

HIGH TEMPERATURE PLASMA CONFINEMENT USING A TRAVEL-LING ELECTROMAGNETIC FIELD Filed Aug. 19, 1958 3 Sheets-Sheet 3 INVENTOR. KARL E. I'IEnNqvIs-r Y BY United States vattent:

This invention relates generally to devices and systems which employ or utilize ionized gaseous media such as,

for example, those utilizing a plasma at high temperatures, and the invention particularly is directed to a new and improved means for and method of confining such a plasma with traveling electric fields.

A plasma is defined as a mass of relatively high but essentially equal densities of lfreely moving positive and negative charge-carrying particles. In a number of instances it is necessary that a plasma be elevated to a high temperature in order for desired reactions to occur. Some of these reactions are chemical in nature and require temperatures of the order of thousands of degrees centigrade while others comprise nuclear reactions which require tempe'atures of a hundred million degrees centigrade and higher. And, in addition to the fact that the plasma must be heated to such high temperatures there is the further requirement that the plasma be thermally isolated from the walls of the chamber or container which houses the plasma.

Heretofore several arrangements have been proposed to eect plasma control. In one instance a pinch effect is employed in which a heavy current is caused to ow through the gas. The heavy current flow results in the production of magnetic lines of 4force which surround and effectively pinch the plasma. This effect is described more fully in an article by R. F. Post entitled Controlled Fusion Research-An Application of the Physics of High Temperature Plasmas appearing in the February 1957 issue of the Proceedings of the IRE, the pertinent portion' commencing at page 143. The pinch effect technique is subject to the disadvantages that it is quite unstable and ions in the gas discharge tend to strike the cathode and anode structures used to produce the discharge and thereby contaminate the magnetic bottle. In another instance a strong axial magnetic field is generated and the plasma particles orbit around the magnetic field lines. And, in a still further arrangement the two systems referred to above have been combined. This results in the generation of a helical magnetic field. The latter two arrangements are subject to the disadvantage that there is some undesired plasma leakage in the axial direction and to the further disadvantage that quite high magnetic field strengths are required (of the order of 100 kilogauss)l which are difficult to achieve in practice.

An object of the invention is to provide an improved means for and method of confining or focusing a quantity of charged particles.

Another object of the present invention is to provide an improved means for and method of thermally isolating a high temperature plasma.

Another object of the invention is to utilize a traveling electric field for confining a gaseous plasma.

A further object of the invention is to provide iniproved means for confining a gaseous plasma by means of electric rather than magnetic fields.

Briciiy, the foregoing and other objects and advantages of theinvention may One or more materials in the gaseous phase are located in a reaction space. The gas is ionized to form a plasma and the plasma is `elevated to a high temperature. traveling wave then is applied to a slow wave structure which at least partially surrounds the reaction space.

3,029,361 Y Patented Apr. 10, 1962 ice 2 when r/wg and E3/WSW 'm Vr, being the maximum potential difference along the slow wave structure, Vt the thermal energy of plasma particles in electron volts, vf the phase velocity of the Atraveling electric wave, m the mass of the heaviest particle in the plasma, and e the charge of this particle, the plasma is properly confined. The requirement that V1.: is much greater than Vt means that the particle velocities caused by the electric field are much greater than the thermal random velocities.

The requirement that pletely confined, the walls of the chamber which houses be achieved in the following manner.

the plasma are thermally isolated from the high temperature plasma.

The invention will be described in greater detail with reference to the accompanying drawing in which:

FIG. l is a view, partly in section taken along the line 1-1 of FIG. 2, of a suitable embodiment of the invention, which embodiment includes a traveling wave helix;

FIG. 2 is a sectional view taken along the line 2-2 of FIG. l;

FIG. 3 is a View, partly in section, taken on the line 3 3 of FIG. l looking in the direction of the arrows;

FIG. 4 is a schematic view that is referred to in explaining the invention;

FIG. 5 is a perspective view of a magic-T waveguide structure through which the traveling wave helix may be energized;

FIGS. 6 and 7 are schematic diagrams illustrating suitable ways of supplying radio frequency energy to the magic-T to produce a traveling wave on the helix;

FIG. 8 isa view illustrating another way in which a traveling wave may be produced on a helix; and

FIG. 9 is a view that is referred to in describing the travelling wave helix.

In FIGS. l, 2 and 3 the drawing is not to scale.

In the several figures similar parts are indicated by similar reference characters.

Refer to FIGURES l, 2 and 3 which illustrate an example of the invention. A reaction chamber 11 is provided which preferably is toroidal in shape and cylindrical in cross-section. The chamber 11 may be formed from any non-magnetic material which withstands high temperatures. Such materials include quartz and other types of glass, and numerous ceramic materials. A valve 13 and conduit 15 further are provided for sealing off the chamber 11, and coupling it either to an exhaust pump or to a gas reservoir.

Ionization of the gas molecules in the chamber 11 to form a plasma, and heating of the plasma to a high temperature may be done by means of a time-varying magnetic field having its lines of force perpendicular to the plane of the toroidal chamber 11. This magnetic eld is provided by a ring-type electro-magnet having a core 17, one leg of which passes through the central aperture of the chamber 11 and another leg of which carries a Winding 21. A bank of capacitors indicated at 19 are charged from a direct-current charging source 18. The desired time-varying magnetic field is produced by closing a switch 20 and thus discharging the capacitor bank 19 through the winding 21.

It will be understood that the capacitor bank discharge through winding 21 is la very high current pulse of short duration. The duration of this pulse and of the resulting time-varying magnetic field may, for example, be one millisecond, or perhaps one-tenth second.

The techniques for supplying the desired high current pulse to the winding 21 are well known. For example, they are known in the radar art where pulsers for high power radar sets charge a capacitor bank or a delay line) through a charging choke, and discharge the capacitor bank into the primary of an output transformer by way of an ignitron switch tube or other mercury pool vapor tube. Techniques of this sort have also been used in operating the well-known betatron.

A travelling Wave helix 23 surrounds the reaction chamber 11. lt may be wound directly on and supported by the chamber 11. Its terminal ends are connected to the inner conductors of separate adjacent coaxial lines 25 .and 27 shielded from each other by means of a thin metal shield 29. The helix 23 comprises a slow wave structure on which a travelling electromagnetic Wave is launched to effect radial confinement of the plasma.

'I'here are various ways of launching a travelling wave on the slow wave structure23. FIGS. 1 and 5 illustrate the use of the well known magic-T waveguide structure for this purpose. The coaxial cables 25 and 27 are fed from the side arms A and B, respectively, of the magic-T. The radio frequency energy may be fed into the two waveguide input arms E and H'either as shown in FIG. 6 or as shown in FIG. 7, for example.

In FIG. 6, two radio frequency generators 41 and 43, which are phase locked, feed energy into the arms E and H, respectively. One of the arms, the B-'arm in the illustration, is fed through a line stretcher 45. The line stretcher is adjusted so that at the side arm A the electric vectors of signals from the E-arrn and the H-arm are in phase, while at the side arm B they are out of phase.

FIG. 7 illustrates a similar feed to the magic-T except that here a single radio frequency generator 47 is used in combination with a directional coupler that feeds half the energy down one waveguide to the E-arm of the magic-T and the other half of the energy down another waveguide to the H-arm.

An advantage of the magic-T feed is tha't it does not require that energy be absorbed in a load.

FIG. 8 illustrates another suitable way of launching a wave on the helix 23, although in this arrangement some energy is lost in a load. Here a radio frequency generator 49 supplies energy to one end of the helix 23. The

other end of the helix is terminated in a non-reiiecting resistor or load 51.

The dimensions of the chamber 11 may be as follows: Mean diameter of the toroid, 1.24 meters; internal diameter of the cross-section of the toroid, 5 centimeters. These figures are given merely by way of example since the toroid 11 may be made in various sizes.V

The toroid 11 and helix 23 may be surrounded by a lithium blanket 31 containing conduits 33 through which a coolant flows for the purpose of utilizing the energy produced by a fusion4 reaction. The blanket 31 and con- 4 within the shells 30 |and 32, by suitable supports not shown. Heat from the lithium will be transferred to the coolant for any desired utilization kof the energy produced by nuclear reactions.

Reference to FIG. 4 will aid in understanding how the travelling wave contines the plasma. In FIG. 4, the solid lines with arrows represent the electric lines of force along the helix at a particular instant. The dotted lines are equipotential lines for the electric field. It will be apparent that there is a succession of electric lenses.

e These lenses function in a manner similar to electrostatic focusingin a cathode ray tube. Assume, for the vpurpose of explanation, thatthe electric lenses are stationary and that an electron is moving through the lenses, left to right. An electron in region A is repelled by the negative eld and forced radially toward the center of the container 11. This occurs while the electron is moving comparatively slowly since it was slowed up as it approached the negative field. When the electron moves into region B, it is pulled outward radially by the positive field. During this period, however, the electron is moving more rapidly since it had been accelerated as it approached the region B. Consequently, the electron is pulled outward radially for less time than it is pushed inward radially. The net effect is that the lens action focuses the electron in the sense that the electron is directed toward the center of the container 11 and away from its walls.

The same action takes place for the ion, a positively charged particle. In this case the ion is repelled in the region B and forced away from the container wall. And

it is in region B that the ion has a comparatively slow axial movement since it was slowed up as it approached the positive field. Again, the net effect is that the lens action directs the charged particle, the ion in this instance. toward the center of the lcontainer 11 and away from itsv walls.

`The foregoing discussion of how the lenses function is intended merely to be an elementary explanation of how electric lenses act tofocus an electron or other charged particle where there is relative motion between the lens and the particle. I'he theory and properties of electric lenses are well known.

Reference will now be made specifically to the way in which applicant contains or focuses a plasma bya travelling wave. Refer again to FIG. 4, and now assume the actual operating conditions that exist in practicing the invention. Under these conditions the container or toroid 11 is filled with plasma consisting of both ions and electrons. These charged particles, i.e. ions andelectrons, have motion, but their speed is low compared with the speed ofthe travelling wave.

Therefore, the charged particles, the plasma, may be considered stationary whereas the electric lenses are moving rapidly through the plasma. The focusing or confinement action for the charged particles is the same as described above where the relative motion of lens and charged particle was assumed to result from motion of the particle ratherA than from motion of the lens.

From the foregoing it will be understood that in FIG. 1 the wave travelling down the helix 23 will confine the plasma in the toroid 11 so that it will be held away from the walls of the toroid. Since the container 11 is ra toroid, it is a container with no ends whereby the problem of the plasma striking the ends of the container is avoided. v v l As previously stated, the travelling wave must move down the helix 23 at a speed that is high compared with the speed of the charged particles of the plasma. The speed of the travelling wave is referred to as the phase velocity vf.

v1=cXtan u' where c is the velocity of light and il: is the pitch angle of the helix. The pitch angle of a helix is illustrated in FIG. 9.

The pitch angle t may be 30 degrees, for example. With this pitch angle the phase velocity centimeters per second. This is the speed of the travelling wave and, therefore, of the electric lenses. It is high compared with the speed of the plasma particles since they will have thermal energy Vt of about 10,000 'electron fvolts, and since the corresponding speed of a plasma particle, specifically a deuterium ion, for example, is

v= 2%25 10s centimeters per second Therefore, in the example given, the speed of the travelling wave is about 170 times the speed of a deuterium ion.

As to the voltage of the radio frequency energy applied to the helix 23, it is such that the maximum potential difference Vr, along the helix is about 100,000 volts, for example. Thus the requirement that Vr, be much greater than V, is satisfied, Vt being about 10,000 electron volts. Also the requirement that is satisfied since g/100,000: 3.1)( 10s centimeters per second and vf=1.7 1010 centimeters per second. Here, again, the calculation is for a deuterium ion. It will be understood that if ions are confined, electrons will also be confined due to space charge forces. l

For a helix with a given pitch xp, and thus a given phase velocity, the frequency of the radio frequency energy applied to the helix preferably should be high enough so that the wave length is of the same order as the helix diameter. Since the phase velocity vf=f where f is the frequency of the radio frequency energy applied to the helix and is the wave length of the travelling wave along the helix, then vf t f It is evident that )t becomes smaller as the frequency f is made larger.

In the example being described, a suitable frequency f for the radio frequency energy applied to the yhelix 23 is 3,000 megacycles per second. This value, as well as the various other values previously given, is given merely by way of example. With the values assumed,

1.7 X 1010 3X 10 Since, as previously stated, the diameter ofthe toroidal chamber 11 is about 5 centimeters, and the helix 23 is wound on chamber 11, it is apparent that A is approximately equal to the helix diameter.

The strength of the magnetic field provided by the magnet 17, 21 may be 20,000 gauss maximum, for example, and have a duration of aboutl millisecond, for example. It may be noted that this time varying magnetic field heats the plasma due tothe toroid of plasma forming, in effect, a shorted secondary of a transformer.

As stated previously, a plasma may be heated to a high temperature either to obtain a chemical reaction or to obtain a nuclear reaction. To obtain a nuclear reaction, deuterium or deuterium and tritium may be introduced into the chamber 11. The sequence of operation may be as follows: y

The valve 13 is actuated to connect the chamber 11 to a pump to pump out the exhausted plasma. Next the valve 13 is actuated to introduce the deuterium or other desired gas into the chamber 11 and then seal the chamber. The radio frequency energy may now be applied A e 6 centimeters 6 to the helix 23, although this energy may be applied to the helix continuously if desired.

Next the capacitor bank 19 is discharged through the winding 21 by closing the switch 20 momentarily. This ionizes and heats the plasma in the chamber 11 to produce the desired reaction. The heated plasma meanwhile is being held away from the walls of the chamber 11 by the travelling wave as has been described. When the reaction is completed, the valve 13 is again actuated to connect the pump to the chamber 11.

The radio frequency input to the helix 23 may be cut ot during the pumping and recharging of the chamber 11, if desired.

During this pumping and recharging period the directcurrent charging source 18 charges the capacitor 19 so it can again be discharged through the winding 21.

If the desired reaction is to be a chemical one, the desired product of the reaction is contained in the chamber 11 and is removed bythe pump after the reaction. In this use of the invention the neutron absorber and heat exchanger may be omitted.

What is claimed is:

l. The method of confining or focusing a quantity of moving charged particles which comprises propagating an electric lens through said quantity of charged particles at a speed that is high compared with the speed of said charged particles.

2. The method of confining a plasma of moving charged particles `in a chamber which comprises propagating a travelling electric field through said plasma at a speed that is high compared with the speed of said charged particles.

3. The method of confining a plasma of moving charged particles in a chamber which .comprises propagating an electric lens through said plasma at a speed that is high compared with the speed of particles in said plasma.

4. Apparatus for controlling a plasma comprising, a chamber containing material in the gaseous phase, means for ionizing and heating said gaseous material to form a high temperature plasma of moving charged particles, and means for confining said plasma comprising means for propagating an electromagnetic wave through said plasma at a speed that is high compared with the speed of said charged particles.

5. Apparatus for controlling a plasma comprising, a chamber containing material in the gaseous phase, means for ionizing and heating said gaseous material to form a high temperature plasma of moving charged particles, and means for confining said plasma comprising means for propagating a travelling electric field through said plasma at a speed that is high compared with the speed of said charged particles.

6. Apparatus for controlling a plasma comprising, a chamber containing material in the gaseous phase, means for ionizing and heating said gaseous material to form a high temperature plasma of moving charged particles, and means for confining said plasma comprising means for propagating an electric lens through said plasma at a speed that is high compared with the speed of said charged particles. Y

7. Apparatus for controlling a plasma comprising, a chamber containing material in the gaseous phase, means for applying a field to said gaseous material to ionize and heat said material to form a high temperature plasma of moving charged particles, and means for confining said plasma comprising means for propagating a travelling electric field periodic in space through said plasma at a. speed that is high compared with the speed of said charged particles.

K 8. Apparatus for controlling a plasma comprising, a chamber containing material inthe gaseous phase, means for applying a time-varying magnetic field to said gaseous material to ionize and heat said material to form a high temperature plasma of moving charged particles, and

means for confining said plasma comprising means for chamber containing material in the gaseous phase, a slow wave propagation structure associated with said chamber, means for ionizing and heating said gaseous material toform a high temperature plasma of moving charged particles surrounded at least in part by said slow wave structure, and means for conning said plasma comprising means for propagating an electromagnetic wave along said slow wave structure and through said plasma at a speed that is high compared with the speed of said charged particles.

10. Apparatus for controlling a plasma comprising, a

toroidal chamber containing material in the gaseous phase,

a slow wave propagating structure associated with said chamber, means for ionizing and heating said gaseous material to form an endless high temperature plasma of moving charged particles surrounded at least in part by said slow wave structure, and means for conning said plasma comprising means for propagating a travelling' electromagnetic wave along said slow wave structure and through said plasma at a speed that is high compared with the speed of said charged particles.

11. Apparatus for controlling a plasma comprising, a chamber containing material in the gaseous phase, a helix surrounding said chamber, means for ionizing and heating said gaseous material to form a high temperature plasma of moving charged particles, and means for conining said plasma comprising means for propagating an electromagnetic wave along said helix and through said plasma at a speed that is high compared with the speed of said charged particles.

12. Apparatus for controlling a plasma comprising, an endless chamber containing material in the gaseous phase, a helix associated with said chamber, means for ionizing and heating said gaseous material to form an endless high temperature plasma of moving charged particles within said helix, and means for confining said plasma comprising vmeans for propagating an electromagnetic wave along said helix and through said plasma at a speed that is high compared with the speed of said charged particles.

13. Apparatus for controlling a plasma comprising, a toroidal chamber containing material in the gaseous phase, a helix surrounding said toroidal chamber, means for ionizing and heating said gaseous material to form an endless high temperature plasma of moving charged particles within said helix, and means for conlining said plasma comprising means for propagating an electromagnetic wave along said helix and through said plasma at a speed that is high compared with the speed of said charged particles.

14. Apparatus -for controlling a plasma comprising, an endless chamber that is to contain material in the gaseous phase, a slow wave propagation structure associated with said chamber, means for applying a time-varying magnetic eld to said gaseous material with the magnetic lines of force encircling said chamber to ionize and heat said material to yform an endless high temperature plasma of moving charged particles within said chamber, and means for confining said plasma comprising means for propagating a travelling electric iield periodic in space along said slow wave propagation structure and through said plasmav at a speed that is high compared with the speed of said charged particles.

15. Apparatus for controlling a plasma comprising, a toroidal chamber that is to contain material in the gaseous phase, a helix associated with said chamber, means for applying a time-varying magnetic teld to said gaseous material with the magnetic lines of force perpendicular to the plane of said toroidal chamber to ionize and heat said material to form an endless high temperature plasma of moving charged particles within said helix, and means Afor confining said plasma comprising means for propagating a travelling electric field periodic in space along said helix and through said plasma at a speed that is high compared with the speed of s aid charged particles.'

16. Apparatus yfor conning'or focusing a quantity of moving charged particles, said apparatus comprising a chamber containing said particles, a slow wave propagation structure associated with said chamber, and means for confining or focusing said particles comprising means for propagating an electromagnetic wave along said slow wave structure and through said quantity of particles, the maximum potential difference along the slow wave strueture being much greater than the thermal energy of said particles in electron volts, and the velocity imparted to the heaviest of said charged particles by the travelling electric field of said wave being less than or equal to the phase velocity of said wave.

17. Apparatus for confining a plasma of moving charged particles comprising, a chamber containing said plasma, a slow wave propagation structure associated with said chamber, and means for confining or focusing said plasma comprising means for propagating an electromagnetic wave along said slow wave structure and through said plasma, the maximum potential difference along the slow wave structure being much greater than the thermal energy of the particles of said plasma in electron volts, and the velocity imparted to the heaviest of the particles of said plasma by the travelling electric eld of said wave being less than or equal to the phase velocity of said wave.

18. Apparatus comprising, a chamber containing fusionable material in the gaseous phase, means for -ionizing and heating said fusionable material to form a high temperature plasma of moving charged particles, means for thermally isolating said high temperature plasma from the walls of said chamber comprising means for propagating an electromagnetic wave through said plasma at a speed that is high compared with the speed of said charged particles, and means for extracting energy from particles released in the thermonuclear reaction resulting from the generation and conlinement of said high temf perature plasma.

19. Apparatus comprising, a chamber containing fusionable material in the gaseous phase, a slow wave propagation structure associated with said chamber, means for ionizing and heating said fusionable material to form a high temperature plasma of moving charged particles, means for thermally isolating said high temperature plasma from the` walls of said chamber comprising means for propagating a travelling electric field periodic in space along said slow wave structure and through said plasma at a speed that is high compared with the speed of said charged particles, and means for extracting energy from particles released in the thermonuclear reaction resulting from the generation and confinement of said high' temperature plasma.

20. Apparatus comprising, an endless chamber containing fusionable material in the gaseous phase, a slow wave propagation structure associated with said chamber, means for applying a time-varying magnetic iield to said fusionable material with the magnetic lines of force encircling said chamber to ionize and heat said material to (References on following page) References Cited in the tile of this patent UNITED STATES PATENTS Pratt June 9, 1936 Haei Dec. 15, 1936 De Forest Nov. 22, 1949 Philos Nov. 21, 1950 Alvarez Mar. 20, 1951 Gorn Apr. 1, 1952 Lerbs Dec. 23, 1952 10 Rothstein Oct. 25, 1955 Elings Nov. 6, 1956 10 Good Nov. 13, 1956 Kilpatrick June 23, 1959 Spitzer Oct. 27, 1959 Josephson Jan. 26, 1960 FOREIGN PATENTS Great Britain July 9, 1952 Great Britain Sept. 21, 1955 OTHER REFERENCES Article by P. C. Thonemann et al., pp. 34-35, Nature, for Ian. 5, 1952. 

